Classically "demons" are identified as deities. Christians worship one true God. If people in the Western world have stopped believing in other deities it could only be because Christianity has had a beneficial influence over the Western mindset.
This thread seems to be primarily revolving around two questions, namely
1) what are demons?, and
2) what can they do?
What are demons (devils)?
Although the NT uses the word “daimonion” to describe them, neither the religious Jews of the period, nor the Christians would have believed for one second in the classical Greek definition of demons as a
divine medium between God and men
, for Scripture is at pains to deny any possibility that such beings exist. For the Jews, the exclusion rests on the basis of Deut 6:4 and similar passages: and the Christians add to the OT’s recognition of only one God, their recognition that there is only one Mediator between God and man the man Christ Jesus (1 Tim. 2:5). It is worth noting that the KJV translators clearly took note of this difference: for whenever a malevolent spiritual being was intended by the writer, (apparently without exception) the KJV men translated the term daimonion as “devil” or “devils” rather than demons. It seems to me that importing the classical definition of demon into a Scriptural discussion is bringing an unnecessary straw man argument to the table, for whatever a “daimon” was to the Greeks, it was something else to the NT authors and religious Jews.
The New Testament uses descriptions by which the people of that time could identify a specific phenomenon. E.g., Acts 16:16; 1 Cor. 10:19-20. At the same time, the phenomena are denied any claim to divine power and honour which men give to them, 1 Cor. 8:4-6. This requires the reader to understand some terms and descriptions in a nominalist sense, Gal. 4:8.
The narratives are certainly accurate, but narrative is not normative and the descriptive is not prescriptive.
While narrative may not always be normative, sometimes it is. What determines whether narrative is normative or not is not an arbitrary principle, but whether the didactic portions of Scripture affirms the conclusions drawn from the narrative. For example, contrary to the Pentecostal claim that Scripture narratives establish the norm that all Christians must speak in tongues, Paul specifically denies that postulate with 1 Cor. 12:30 while norming the deductions that all Christians will have the Holy Spirit and must go on being filed with the Holy Spirit. In the matter here discussed, the proposition that biblical devils are personal spirits is shown by several narrative incidents which record the devils speaking, including Matt 8:31, and Mark 5:9, 15, Mark 7, 25, 26. The latter passages are particularly significant as they identify “unclean spirits” with “devils, from which one may draw the conclusion that the Scriptural “devils” are personal spirits. The personality of devils is confirmed didactically confirmed by James 2:19 in which the inspired author claims that they “believe and shudder” at the recognition that God is one. Anyone who wants to deny the reality of such personal evil spirits from Scripture will need to present a case from Scripture’s didactic material.
No Christian reader of Acts 16:16 would ever propose that the slave girl had divine power or was worthy of divine honour. Calvin’s nominalism in his comments on this passage would appear to be limited to the word “fortunetelling.” I hope by “nominalist sense” you are not denying the reality of the “spirit of divination” that was afflicting the girl.
What can the devils do?
Whatever they do is within God’s limits.
Can they possess people?
In the NT we see reports of "possession" written in a way as observed by the superstitious thinking of that time as we see in the testimonies of people today.
The NT accounts depict Christ, St. Paul and the inspired authors of the Gospels / Acts recognizing the reality of devil possession without a single sentence anywhere in the didactic portions of scripture calling the really of such possession into question. Absent a Scriptural case to the contrary, I think we have to take the possibility of demon possession of unbelievers as a given. (Which is not to say that the devils can “possess” Christians. 1 John 4:4 is enough to establish that they cannot do so although they might try to deceive an unwary believer into thinking that such a condition is possible.)
They can foretell to a limited extent, if Calvin is correct in stating that God grants “Satan so great liberty, as to suffer him to deceive miserable men, and to bewitch them with true divinations,” foretell part of the future although that knowledge is given by God. Please note that such Divine action if it occurs does not remove
the medium of demons altogether
. Rather, this is another example where God uses the demon, by presenting the apparent foretelling from a non-Divine source, to accomplish his purpose, namely the hardening of men in sin.
Can they tempt men and women?
While Scripture (James 1:14) makes it plain that the primary source of temptation is one’s own sinful nature, Scripture is also clear that secondary external sources of temptation exist (Prov. 1:10) and that we have to watch out for both. If devilish enticements are a reality, they in no way absolve us of our responsibility if we give in to them, just as the reality of human enticements does not absolve us of our responsibility to walk in God’s ways.